1. Field of the Invention
The present invention relates to a property monitoring apparatus for accurately monitoring the property of a current transformer or an electric transformer used for example in a measurement device, such as a zero-phase current transformer (ZCT) used in an insulation monitoring apparatus.
2. Description of the Related Art
Patent document 1 (Japanese patent application laid-open publication No. 2003-215196) and patent document 2 (Japanese patent application laid-open publication No. 2002-289085) disclose arts related to measuring a leak current in an insulation monitoring apparatus using a zero-phase current transformer (ZCT), wherein the output of the zero-phase current transformer (ZCT) is taken in to measure the leak current value.
There are multiple variations of turns and gauges in zero-phase current transformers (ZCT), and the properties differ among respective transformers. In order to realize accurate measurement, it is necessary to consider the property of the specific component and to correct the measurement value based on the property in order to obtain an accurate measurement value.
According to the prior art insulation monitoring apparatus, the property of the individual zero-phase current transformer to be used is measured in advance, and the correction value thereof is stored in a storage section of the apparatus, based on which the measurement value is corrected for a more accurate measurement. However, the zero-phase current transformers (ZCT) that can be used are restricted to those having obtained such correction values in advance.
Patent document 1 discloses superposing a test current of a fixed level having a frequency that differs from the leak current to be measured, to thereby measure the rate of change thereof so as to obtain the above-mentioned correction value.
Patent document 2 discloses outputting a test current to a tertiary winding of a zero-phase current transformer so as to generate a simulated leak current to the circuit, so as to compare the output from a secondary winding of the zero-phase current transformer in response to the simulated leak current with a theoretical value of the simulated leak current and obtain the correction value based on the difference.
Patent document 3 (Japanese patent application laid-open publication No. 10-153625) discloses a general input current detection circuit, wherein the detection value of an input current flowing to a primary side of a current transformer is corrected by the ratio of the detection value detected by flowing a reference current.
According to the prior art insulation monitoring apparatus, as described earlier, the zero-phase current transformers (ZCT) as measurement media have respectively varied properties, so that the properties of the respective zero-phase current transformers (ZCT) must be measured in advance and the correction value thereof must be stored in a storage section of the apparatus, and therefore, it is not possible to use zero-phase current transformers other than those having their properties measured in advance.
Further according to the prior art insulation monitoring apparatus, the highly accurately controlled test current is not directly passed through the zero-phase current transformer (ZCT), and therefore, the property of the zero-phase current transformer (ZCT) cannot be directly measured. Moreover, the prior art apparatus does not allow confirmation of the state of connection regarding the zero-phase current transformer being used and the operation of the leak current detection circuit in a no-load state during periodic inspection.
Therefore, the present invention aims at providing a measurement device having a correction function capable of obtaining an accurate correction value by measuring the properties of the current transformer or the voltage transformer incorporated in the measurement device of an insulation monitoring apparatus or the like regardless of the type of current transformer or voltage transformer being adopted, and capable of confirming the operation of the measurement device in the state of connection with the current transformer or the voltage transformer being used and in the no-load state during periodic inspection.